Heavy duty tire with tread portion having three longitudinal main grooves

ABSTRACT

A heavy duty tire has a tread portion  2  which is divided into five inner, intermediate and outer rib-like land portions R 1 , R 2 , R 2 , R 3  and R 3 . The inner rib-like land portion R 1  is virtudally divided into half regions R 1   a  and R 1   a  on each side of a tire equator. The intermediate rib-like land portions R 2  and R 3  are virtually divided into tire equator-side half regions R 2   c  and R 3   c , and into ground-contact edge-side half regions R 2   e  and R 3   e . In a regular ground-contact state in which a normal load is applied, when total sums of ground-contact load applied to the half regions R 1   a , R 2   c , R 2   e , R 3   c  and R 3   e  are defined as P 1   a , P 2   c , P 2   e , P 3   c  and P 3   e,  
         P 2   c /P 1   a  is set to 0.9 to 1.05,   P 2   e /P 2   c  is set to 0.75 to 1.0,   P 3   c /P 2   e  is set to 0.9 to 1.2, and   P 3   e /P 3   c  is set to 0.8 to 1.1.

TECHNICAL FIELD

The present invention relates to a heavy duty tire for suppressinguneven wear and equalizing wear by specifying a distribution of a totalsum of ground-contact force.

BACKGROUND TECHNIQUE

In the case of a heavy duty tire for example, generally, its treadprofile shape (a) is formed into a single arc shape in a vulcanizationdie as schematically shown in FIG. 7.

In a regular internal pressure state in which such a tire is mounted ona regular rim and a regular internal pressure is charged into the tire,however, there is a tendency that a tread surface swells radiallyoutward in a region Y separated from a tire equator by a distance whichis 0.5 to 0.7 times a half of a ground-contact width of the tread.Therefore, a circumferential length difference between the swellingportion (b) and the tread ground-contact edge (e) becomes great, a slipis generated between the tread surface on the side of the treadground-contact edge and a road surface, and uneven wear such asso-called unbalanced wear is prone to be generated.

Hence, in order to suppress the unbalanced wear, as disclosed inJapanese Patent Application Laid-open No. H7-164823, in a tread profileshape, it is proposed that a tread ground-contact edge portion(so-called tread shoulder portion) is formed of a flat arc having agreat radius of curvature as compared with a tire equator portion(so-called tread center portion), and a ground-contact length in aground-contact surface shape of the tread shoulder portion is elongated.

According to this technique, however, rubber gauge thickness in thetread shoulder portion is increased, and excessive increase in theradius of curvature brings about disadvantage because accumulated heatrises a temperature and a belt end is peeled off. From this point ofview, there is a limit for increasing the radius of curvature and asuppressing effect of unbalanced wear can not sufficiently be exhibited.

The present inventor focused attention on a relation between theground-contact force and the uneven wear and studied the relation. As aresult, the inventor found that a correlation between the ground-contactforce and wear energy was strong, and if the distribution of theground-contact force was specified, it was possible to suppress theuneven wear including the unbalanced wear and to equalize the wearwithout excessively increasing the rubber gauge thickness.

That is, it is an object of the present invention to provide a heavyduty tire capable of suppressing the uneven wear including theunbalanced wear and equalizing the wear without excessively increasingthe rubber gauge thickness.

DISCLOSURE OF THE INVENTION

To achieve the above object, in a heavy duty tire comprising a carcassextending from a tread portion to a bead core of a bead portion througha sidewall portion, and a belt layer disposed inside the tread portionand outside the carcass, the tread portion is divided by three or fourlongitudinal main grooves extending in a circumferential direction ofthe tire into four or five rib-like land portions, the present inventionis characterized in that

when a normal load is applied to a tire in its normal ground-contactstate in which a regular rim is assembled into the tire and a regularinternal pressure is charged into the tire, in a total sum ofground-contact force applied to a tire equator side half region and aground-contact edge side half region, each of which being a rib-likeland portion, a ratio of a total sum of the ground-contact force betweenthe axially adjacent half regions is limited to the followingpredetermined ranges.

That is, in a heavy duty tire in which the tread portion is divided intofive rib-like land portions, i.e., an inner rib-like land portion R1 ona tire equator, ground-contact edge-side outer rib-like land portionsR3, and intermediate rib-like land portions R2, a first invention ischaracterized in that

a ratio P2 c/P1 a between a total sum P1 a of ground-contact forceapplied to a half region R1 a obtained by dividing an inner rib-likeland portion R1 by the tire equator and a total sum P2 c ofground-contact force applied to a tire equator side half region R2 c inthe intermediate rib-like land portion R2 is set in a range of 0.9 to1.05,

a ratio P2 e/P2 c between the total sum P2 c of the ground-contact forceand a total sum P2 e of ground-contact force applied to a ground-contactedge side half region R2 e in the intermediate rib-like land portion R2is set in a range of 0.75 to 1.0,

a ratio P3 c/P2 e between the total sum P2 e of the ground-contact forceand a total sum P3 c of ground-contact force applied to a tire equatorside half region R3 c in the outer rib-like land portion R3 is set in arange of 0.9 to 1.2, and

a ratio P3 e/P3 c between the total sum P3 c of the ground-contact forceand a total sum P3 e of ground-contact force applied to a ground-contactedge side half region R3 e in the outer rib-like land portion R3 is setin a range of 0.8 to 1.1.

In a heavy duty tire in which a tread portion is divided into fourrib-like land portions J, i.e., inner rib-like land portions J1 on eachside of a tire equator and ground-contact edge-side outer rib-like landportions J2, a second invention is characterized in that

a ratio P1 e/P1 c between a total sum P1 c of ground-contact forceapplied to a tire equator-side half region J1 c in the inner rib-likeland portion J1 and a total sum P1 e of ground-contact force applied toa ground-contact edge-side half region J1 e is set in a range of 0.8 to1.0,

a ratio P2 c/P1 e between the total sum P1 e of the ground-contact forceand a total sum P2 c of ground-contact force applied to a tire equatorside half region J2 c in the outer rib-like land portion J2 is set in arange of 0.8 to 1.0, and

a ratio P2 e/P2 c between the total sum P2 c of the ground-contact forceand a total sum P2 e of ground-contact force applied to a ground-contactedge side half region J2 e in the outer rib-like land portion J2 is setin a range of 0.6 to 1.0.

In this specification, the “regular rim” is a rim determined for eachtire according to a standard on which the tire is based among standardsfor tires, and for example,

-   -   in JATMA, when a narrower rim width than the standard rim is        set, the regular rim for the tire is a “rim having a rim width        which is narrower than the standard rim by one rank”, and when        no narrower rim width than the standard rim is set, the regular        rim for the tire is a “standard rim”,    -   in TRA, when a narrower rim width than the “Design Rim” is set,        the regular rim for the tire is a “rim having narrower width        than the “Design Rim” by one rank”, and when no narrower rim        width than the “Design Rim” is set, the regular rim for the tire        is the “Design Rim”, and    -   in ETRTO, when a rim width narrower than the “Measuring Rim” is        set, the regular rim for the tire is a “rim having narrower        width than the “Measuring Rim” by one rank”, and when no        narrower rim than the “Measuring Rim” is set, the regular rim        for the tire is the “Measuring Rim”.

Further, the “regular internal pressure” means an air pressuredetermined for each tire according to a standard on which the tire ofthe present invention is based among standards for tires. The regularinternal pressure means a maximum air pressure in the case of JATMA, theregular internal pressure means a maximum value described in “TIRE LOADLIMITS AT VARIOUS COLD INFLATION PRESSURES” in the case of TRA, and theregular internal pressure means a “INFLATION PRESSURE” in the case ofETRTO, but when the tire is for a passenger vehicle, the regularinternal pressure is 180 kPa. The “normal load” means a load determinedfor each tire according to a standard on which the tire of the presentinvention is based among standards for tires. The normal load is amaximum load capacity in the case of the JATMA, and the normal loadmeans a maximum value described in the Table “TIRE LOAD LIMITS ATVARIOUS COLD INFLATION PRESSURES” in the case of TRA, and the normalload means “LOAD CAPACITY” in the case of ETRTO.

In this specification, the “ground-contact edge” is an outer edge in thetire axial direction of the tread ground-contact surface which comesinto contact with the ground when the normal load is applied to the tirein the regular internal pressure state in which the normal rim isassembled to the tire and the regular internal pressure is charged intothe tire. A distance between the outer edge (ground-contact edge) andthe tire equator is called half of the ground-contact width of thetread.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a tire of an embodiment of the presentinvention.

FIG. 2 is an enlarged sectional view of a tread portion of a tire of afirst invention.

FIG. 3 is an enlarged sectional view of a tread portion of a tire of asecond invention.

FIG. 4 is a diagram showing a distribution of a total sum of aground-contact force applied to each half region in Table 1.

FIG. 5 is a diagram showing a distribution of a total sum of aground-contact force applied to each half region in Table 2.

FIG. 6 is a diagram for explaining an evaluation method of orbital wearand unbalanced wear.

FIG. 7 is a diagram showing a tread profile shape in a conventionaltire.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below togetherwith illustrated examples. FIG. 1 is a sectional view of a heavy dutytire of the invention for a truck, a bus and the like.

In FIG. 1, a heavy duty tire 1 comprises a carcass 6 extending from atread portion 2 to a bead core 5 of a bead portion 4 through a sidewallportion 3, and a belt layer 7 disposed inside the tread portion 2 andoutside the carcass 6.

The carcass 6 comprises one or more (one, in this example) carcass ply6A in which a carcass cord is arranged at an angle of 70 to 90° withrespect to a circumferential direction of the tire. A metal cord such assteel is used as the carcass cord.

The carcass ply 6A has a ply body 6 a extending between the bead cores 5and 5. The ply body 6 a is provided at its opposite sides with turnupportions 6 b which are secured after being turned up from inner sidetoward outer side around the bead core 5. A bead apex rubber 8 extendingradially outward from the bead core 5 is disposed between the ply body 6a and the turnup portion 6 b. The bead apex rubber 8 reinforces aportion of the tire from the bead portion 4 to the sidewall portion 3.

The belt layer 7 comprises three or more belt plies using metal cords asbelt cords. In this example, the belt layer 7 comprises four belt plies,i.e., a first belt ply 7A in which a steel cord is arranged radiallyinnermost side at an angle of 60±15° for example with respect to thecircumferential direction of the tire, and second to fourth belt plies7B, 7C and 7D in which steel cords are arranged at a small angle of 10to 35° for example with respect to the circumferential direction of thetire.

In the belt layer 7, a ply width of the first belt ply 7A in the axialdirection of the tire is smaller than a ply width of the second belt ply7B and is substantially equal to a ply width of the third belt ply 7C. Aply width WB of the second belt ply 7B having the greatest width is 0.80to 0.95 times a tread ground-contact width WT, thereby reinforcingsubstantially the entire width of the tread portion 2 while providing ahoop effect and enhancing the rigidity of the tread. The fourth belt ply7D having the narrowest width functions as a breaker which protects thefirst to third belt plies 7A to 7D and the carcass 6 from damage.

Next, the tread portion 2 of the heavy duty tire 1 of the firstinvention comprises four longitudinal main grooves G, i.e., innerlongitudinal main grooves G1 extending on opposite sides of the tireequator C and outer longitudinal main grooves G2 extending outer sidesof the longitudinal main grooves G1. With this structure, a treadsurface is divided into five rib-like land portions R, i.e., a rib-likeland portion R1 located at an inner side on the tire equator C, rib-likeland portions R3 located at outer sides of the ground-contact edge E,and rib-like land portions R2 located between the rib-like land portionsR1 and R3. The rib-like land portions R may be block rows or a rib.

Each of the longitudinal main grooves G1 and G2 has a groove width of 3mm or more. The longitudinal main groove G is straight or zigzag(including corrugate) in shape and extends in the circumferentialdirection. It is preferable that the groove width of each of thelongitudinal main grooves G1 and G2 is 5 mm or more, and more preferablyin a range of 7 to 10 mm. It is preferable that a groove depth is 9 mmor more, and more preferably in a range of 14.5 to 17.5 mm.

In this example, a groove center line N of the outer longitudinal maingroove G2, i.e., the shoulder groove Gs located at the outermost side inthe axial direction of the tire, passes through a region Y which isseparated from the tire equator C by a distance of 0.5 to 0.7 times thehalf of the ground-contact width of the tread WT/2. With this design,the tread portion 2 is divided into a tread center portion Yc located onan inner side of the shoulder groove Gs and a tread shoulder portion Yslocated on an outer side of the shoulder groove Gs. The inner andintermediate rib-like land portions R1 and R2 are disposed in the treadcenter portion Yc, and the outer rib-like land portion R3 is disposed inthe tread shoulder portion Ys. When the shoulder groove Gs is a zigzaggroove, a center of the zigzag amplitude is the groove center line N.

In the heavy duty tire 1 of the first invention, in order to suppressthe uneven wear in the tread pattern of each of the five ribs, and toequalize the wear, a normal rim is assembled to the tire 1 and a regularinternal pressure is charged into the tire. A normal load is applied tothe tire in the regular internal pressure state and the tire is broughtinto a normal ground-contact state. The ground-contact force in thisstate is specified as follows:

As shown in FIG. 2 in detail, when the rib-like land portion R1 isvirtually divided into half regions R1 a and R1 a on each side of thetire equator C, and the intermediate and outer rib-like land portions R2and R3 are virtually divided into tire equator side half regions R2 cand R3 c and ground-contact edge side half regions R2 e and R3 e,

1) a ratio P2 c/P1 a between a total sum P1 a of ground-contact forceapplied to the half region R1 a of the rib-like land portion R1 and atotal sum P2 c of ground-contact force applied to the tire equator sidehalf region R2 c in the intermediate R2 is set in a range of 0.9 to1.05,

2) a ratio P2 e/P2 c between the total sum P2 c of the ground-contactforce and a total sum P2 e of the ground-contact force applied to theground-contact edge side half region R2 e in the rib-like land portionR2 is set in a range of 0.75 to 1.0,

3) a ratio P3 c/P2 e between the total sum P2 e of the ground-contactforce and a total sum P3 c of the ground-contact force applied to thetire equator side half region R3 c in the outer rib-like land portion R3is set in a range of 0.9 to 1.2, and

4) a ratio P3 e/P3 c between the total sum P3 c of the ground-contactforce and a total sum P3 e of the ground-contact force applied to theground-contact edge side half region R3 e in the outer rib-like landportion R3 is set in a range of 0.8 to 1.1.

Each total sum P1 a, P2 c, P2 e, P3 c and P3 e of the ground-contactforce can be obtained in the following manner. That is, the tire 1 towhich the normal load is applied is brought into contact with a sheetbody on which a large number of sensors are spread, and a load appliedto each the sensor is measured. Outputs of the sensors with which thehalf regions contacted are summed for every half region, and it ispossible to obtain the total sum of the ground-contact force applied tothe half regions R1 a to R3 e.

Here, the present inventor found that the total sum of theground-contact force has a strong correlation with respect to the wearenergy, and if the total sum P1 a to P3 e of the ground-contact forceapplied to the half regions R1 a to R3 e fall within the ranges 1) to4), the uneven wear including the unbalanced wear could be suppressedand the wear could be equalized.

Especially in the 1) to 3), if the ratios are out of the above ranges,balance of wear energies between the half regions R1 a to R3 e is lost,and there is a tendency that orbital wear or punching wear is generated.In the 4), if the ratio P3 e/P3 c is out of the range of 0.8 to 1.1,there is a tendency that the unbalanced wear is generated.

From a view point of equalization of wear in the entire tread portion 2,it is preferable that the ratio P3 e/P1 a between the total sum P1 a ofthe ground-contact force and the total sum P3 e of the ground-contactforce is set in a range of 0.75 to 1.0.

Next, in order to obtain the distribution of the total sum of theground-contact force, in this example, when a tread thickness betweenthe second belt ply 7B and the profile line S (tread profile line S,hereinafter) of the tread surface in the regular internal pressure stateas shown in FIG. 2,

-   -   a smallest tread thickness position Qt where the tread thickness        T becomes a minimum value Tmin is provided in the region Y,    -   the minimum value Tmin is set to 0.89 to 0.97 times the tread        thickness Tc at the tire equator C, and    -   a tread thickness Tb of the second belt ply 7B at the outer end        position is set to 0.95 to 1.10 times the tread thickness Tc.

At that time, it is preferable that the tread thickness T is graduallyincreased from the smallest tread thickness position Qt axially inwardof the tire to the tread thickness Tc, and axially outward of the tireto the tread thickness Tb.

By employing such a distribution of the tread thickness T, it ispossible to obtain the distribution of the total sum of theground-contact force. With this feature, the tread thickness Tb becomesequal to or less than 1.10×Tc, increase in the rubber gauge thickness inthe tread shoulder portion Ys can be suppressed, the separation of theend of the belt by the heat of rubber can be prevented, and highendurance can also be ensured.

In this example, in order to obtain the distribution of the treadthickness T, as shown in FIG. 1, the second belt ply 7B is formed of asingle arc whose center is on the tire equator C, the tread profile lineS in the tread center portion. Yc is formed of a projecting arc profileline S1 using a single arc or a plurality of arcs, and the tread profileline S in the tread shoulder portion Ye is formed of a substantiallystraight profile line S2.

Next, a heavy duty tire 1 of a second invention will be explained basedon FIG. 3.

The tread portion 2 of the heavy duty tire 1 of the second inventioncomprises three longitudinal main grooves G, i.e., a centrallongitudinal main groove G1 extending on the tire equator C, and twoouter longitudinal main grooves G2 extending on each side of thelongitudinal main groove G1. With this structure, the tread surface isdivided into four rib-like land portions J, i.e., inner rib-like landportions J1 on each side of the tire equator C, and outside rib-likeland portions J2 on the ground-contact edge E side. The rib-like landportions J may be block rows or a rib, like the rib-like land portionsR. The shoulder groove Gs which is the outer longitudinal main groove G2is formed in the region Y.

In the heavy duty tire 1 of the second invention, in order to suppressthe uneven wear and equalize the wear in the tread patterns with fourribs like this, the ground-contact length in the normal ground-contactstate is specified as follows:

As shown in FIG. 3 in detail, when the inner rib-like land portion J1 isvirtually divided into a tire equator side half region J1 c and aground-contact edge side half region J1 e, and the outer rib-like landportion J2 is virtually divided into a tire equator side half region J2c and a ground-contact edge side half region J2 e,

1) in the inner rib-like land portion J1, a ratio P1 e/P1 c between anaverage P1 c of the ground-contact force of the tire equator side halfregion J1 c and an average P1 e of the ground-contact force of theground-contact edge side half region J1 e is set in a range of 0.8 to1.0,

2) a ratio P2 c/P1 e between a total sum P1 e of the ground-contactforce and a total sum P2 c of the ground-contact force applied to thetire equator side half region J2 c in the outer rib-like land portion J2is set in a range of 0.8 to 1.0, and

3) a ratio P2 e/P2 c between the total sum P2 c of the ground-contactload and a total sum P2 e of the ground-contact load applied to theground-contact edge side half region J2 e is set in a range of 0.6 to1.0.

Each total sum P1 c, P1 e, P2 c and P2 e can be obtained in the samemanner as that of the first invention.

As a result of research of the present inventor, in the case of treadpatterns with four ribs, it was found that if the total sums P1 c, P1 e,P2 c and P2 e of the ground-contact force are set in the range of the 1)to 3) such that the total sums are uniformly reduced from the tireequator C toward the tread ground-contact edge E (P1 c≧P1 e≧P2 c≧P2 e),uneven wear including unbalanced wear could be suppressed and the wearcould be equalized.

That is, if the ratio P1 e/P1 c is limited to 0.8 to 1.0 as shown in 1),uneven wear on the ground-contact edge side in the inner rib-like landportion J1 can be suppressed. If the ratio P2 e/P2 c is limited to 0.6to 1.0 as shown in 3), uneven wear on the ground-contact edge side inthe outer rib-like land portion J2 can be suppressed. If the ratio P2c/P1 e is limited to 0.8 to 1.0 as shown in 2), uneven wear on the tireequator side of the outer rib-like land portion J2 with respect to theinner rib-like land portion J1 can be suppressed.

If the ratio P1 e/P1 c, ratio P2 e/P2 c and ratio P2 c/P1 e are out ofthe above ranges, balance of wear energies of the half regions J1 c toJ2 e is lost, and uneven wear is generated. Especially in order toenhance the suppressing effect of unbalanced wear, it is preferable thatthe ratio P2 e/P2 c is set to 0.7 or higher.

From a point of view of equalization of wear in the entire tread portion2, it is preferable that the ratio P2 e/P1 c between the total sum P1 cof the ground-contact force and the total sum P2 e of the ground-contactforce is set in a range of 0.4 to 0.9.

Next, in order to obtain such a distribution of the total sum of theground-contact force, in this example, when a tread thickness betweenthe profile line S of the tread surface in the regular internal pressurestate (tread profile line S, hereinafter) and the second belt ply 7B isdefined as T as shown in FIG. 2,

-   -   a tread thickness Ty in each position in the region Y is set in        a range of 0.91 to 1.05 times the tread thickness Tc at the        position of the tire equator C, and    -   a tread thickness Tb at the position of the outer end of the        second belt ply is set in a range of 0.98 to 1.03 times the        tread thickness Tc.

By employing such a distribution of the tread thickness T, it ispossible to obtain the distribution of the total sum of theground-contact force. With this feature, the tread thickness Tb becomesequal to or less than 1.03×Tc, increase in the rubber gauge thickness inthe tread shoulder portion Ys can be suppressed, the separation of theend of the belt by the heat of rubber can be prevented, and highendurance can also be ensured.

In this example, in order to obtain the distribution of the treadthickness T, as shown in FIG. 3, the second belt ply 7B is formed of asingle arc whose center is on the tire equator C, the tread profile lineS in the tread center portion Yc is formed of a projecting arc profileline S1 using a single arc or a plurality of arcs, and the tread profileline S in the tread shoulder portion Ye is formed of a substantiallystraight profile line S2.

Although the preferred embodiments of the present invention weredescribed above, the invention is not limited to the illustratedembodiments, and can be deformed in various modes.

Embodiment A

The heavy duty tires (tire size is 295/80R22.5) of the first inventionhaving the internal structure shown in FIG. 1 were prototyped based onspecs shown in Table 1, and wear of the prototyped tires was tested. Aresult of the test is shown in Table 1. FIG. 4 is a diagram showing adistribution of the total sum of the ground-contact force applied toeach the half region.

(1) Wear;

Rims (22.5×9.00) were assembled to the tread portion, internal pressure(850 kPa) was charged into the tires, and tires were mounted to frontwheels of a truck (2-2·D type), the truck was allowed to run through adistance of 100,000 km. In the tires after running, the wear amounts ofthe following positions were measured:

-   -   a side edge position j1 in the ground-contact edge-side of the        inner rib-like land portion R1,    -   a side edge position j2 in the tire equator-side of the        intermediate rib-like land portion R2,    -   a side edge position j3 in the ground-contact edge-side of the        intermediate rib-like land portion R2,    -   a side edge position j4 in the tire equator-side of the outer        rib-like land portion R3, and    -   a side edge position j5 in the ground-contact edge-side of the        outer rib-like land portion R3 (corresponding to the        ground-contact edge E).

The wear amount are indicated while a comparative example 1 is definedas 100. The smaller the value, the smaller the wear amount is.

TABLE 1 Embodiment Embodiment Embodiment Embodiment ComparativeComparative Comparative A1 A2 A3 A4 example A1 example A2 example A3Tread ground- 240 240 240 240 240 240 240 contact width WT <mm> Width WBof belt ply 220 220 220 220 220 220 220 (mm) Ground-contact force (KN)P1a 3.14 3.43 3.30 3.33 3.45 3.11 3.45 P2c 3.22 3.19 3.22 3.10 3.38 3.063.25 P2e 2.94 2.87 2.71 2.79 3.21 2.90 2.37 P3c 3.24 2.93 3.03 2.94 2.892.90 2.96 P3e 2.7 2.81 2.98 3.08 2.31 3.27 3.20 Total 15.24 15.24 15.2415.24 15.24 15.24 15.24 Ratio of ground- contact force P2c/P1a 1.03 0.930.98 0.93 0.98 0.98 0.94 P2e/P2c 0.91 0.90 0.84 0.90 0.95 0.95 0.73P3c/P2e 1.10 1.02 1.12 1.05 0.90 1.00 1.25 P3e/P3c 0.83 0.96 0.98 1.050.80 1.13 1.08 P3e/P1a 0.86 0.82 0.90 0.93 0.67 1.05 0.93 Treadthickness T <mm> Tmin (*1) 22.75 23.5 23.5 23.5 25 23.5 22.0 Tc 25 25 2525 25 25 25 Tb 25 24.25 25.25 26.75 25 28.75 25 (Ratio Tmin/Tc) 0.91 940.94 0.94 1 0.94 0.88 (Ratio Tb/Tc) 1 0.97 1.01 1.07 1 1.15 1 Wear Wearamount 100 100 100 100 100 100 105 (position j1) Wear amount 100 99 100100 100 105 110 (position j2) Wear amount 100 98 99 100 100 105 95(position j3) Wear amount 95 96 96 95 100 100 95 (position j4) Wearamount 87 90 85 80 100 90 95 (position j5) (*1) A distance from the tireequator to the smallest tread thickness position Qt is 0.55 times thehalf of the tread ground-contact width: WT/2. (Others) Distances fromthe tire equator to groove center lines of the inner and outerlongitudinal main grooves are 0.175 times and 0.558 times the half ofthe tread ground-contact width: WT/2, respectively. The radius ofcurvature of the belt ply is 580 mm.

Embodiment B

The heavy duty tires (tire size is 11R22.5) of the second inventionhaving the internal structure shown in FIG. 2 were prototyped based onspecs shown in Table 2, and wear of the prototyped tires was tested. Aresult of the test is shown in Table 2. FIG. 5 is a diagram showing adistribution of the total sum of the ground-contact force applied toeach half region in the embodiment B1 and the comparative examples B1and B2.

(1) Wear;

Rims (22.5×7.50) were assembled to the tread portion, internal pressure(800 kPa) was charged into the tires, and tires were mounted to frontwheels of a truck (2-2·D type), the truck was allowed to run through adistance of 10,000 km. In the tires after running,

(a) a wear amount Z1 in the center longitudinal main groove G1 and awear amount Z2 in the outer longitudinal main groove G2 (shoulder grooveGs) were measured, and ratios Z1/Z2 were compared. If the ratio Z1/Z2 isgreater than 1.0, there is a tendency that center wear is generated, andif the ratio z1/z2 is smaller than 1.0, there is a tendency thatshoulder wear is generated, and if the ratio Z1/Z2 is closer to 1.0, theequalization of wear is more excellent. The wear amount Z1 is an averagevalue of the wear amount in both groove-side edges of the centerlongitudinal main groove G1, and the wear amount Z2 is an average valuein both the groove-side edges of the outer longitudinal main groove G2.

(b) As shown in FIG. 6, a drop amount Z3 of the side edge G2 c in thetire equator-side of the outer longitudinal main groove G2 with respectto a reference arc RR passing through the tire equator C and the sideedges G2 e and G2 e in the ground-contact edge-side of the outerlongitudinal main groove G2 was measured and compared. The greater thevalue, the orbital wear is greater. A drop amount Z4 of the treadground-contact edge E with respect to the reference arc RR was measuredand compared. The greater the value, the greater the unbalanced wear is.

TABLE 2 Comparative Comparative Embodiment B1 Embodiment B2 example B1example B2 Tread ground-contact 225 225 201 225 width WT <mm> Width WBof belt ply 200 200 192 188 <mm> Ratio of ground- contact force P1e/P1c0.844 0.868 0.743 0.859 P2e/P2c 0.789 0.697 0.557 0.559 P2c/P1e 0.8010.809 1.060 0.720 Tread thickness T Ty(*1) 25 26.1 26.33 26.95 Tc 25 2525 25 Tb 25.73 25.55 26.85 26.73 (Ratio Ty/Tc) 1.000 1.044 1.053 1.078(Ratio Tb/Tc) 1.029 1.022 1.074 1.069 Wear Ratio Z1/Z2 1.15 1.44 1.191.05 Z3 0.0 0.0 2.0 0.5 Z4 1.5 2.0 5.0 3.5 *1 Measured at a positionseparated away from the tire equator by a distance of 0.6 times the halfof the tread ground-contact width: WT/2. * (Others) A distance from thetire equator to the groove center line of the outer longitudinal maingroove is 0.54 times the half of the tread ground-contact width: WT/2,and a groove width is 12 mm. A radius of curvature of the belt ply is580 mm.

As shown in Tables 1 and 2, according to each of the tires of theembodiments of the first and second inventions, when a normal load isapplied to the tire in its regular internal pressure state, in the totalsum of the ground-contact force applied to the tire equator side halfregion and the ground-contact edge side half region, each of which beingthe rib-like land portions, since the ratio of the total sum of theground-contact force between the axially adjacent half regions isspecified, it is possible to suppress the uneven wear including theunbalanced wear and to equalize the wear without excessively increasingthe rubber gauge thickness.

INDUSTRIAL APPLICABILITY

Since the heavy duty tire of the present invention has theabove-described structure, it is possible to suppress the uneven wearincluding the unbalanced wear and to equalize the wear withoutexcessively increasing the rubber gauge thickness.

1. A heavy duty tire comprising a carcass extending from a tread portionto a bead core of a bead portion through a sidewall portion, and a beltlayer disposed inside the tread portion and outside the carcass, whereinthe belt layer includes a carcass-side first belt ply and a second beltply disposed outside thereof, and when the tread thickness between theprofile line of the tread surface and the second belt ply is defined asT, a tread thickness Ty in a region Y the boundaries of which areseparated from the tire equator C by distances of 0.5 and 0.7 times thehalf of the ground-contact width of the tread is set in the range of1.00 to 1.05 times the tread thickness Tc at a position of the tireequator C, and the tread thickness Tb at the position of the outer endof the second belt ply is set in a range of 0.98 to 1.03 times the treadthickness Tc, the tread portion is divided by three longitudinal maingrooves extending in a circumferential direction of the tire into innerrib-like land portions J1 on each side of the tire equator andground-contact edge-side outer rib-like land portions J2, and eachlongitudinal main groove between the inner rib-like land portion J1 andthe outer rib-like land portion J2 is located in said region Y, in aregular ground-contact state in which the tire is mounted on the regularrim, the regular internal pressure is charged into the tire and thenormal load is applied to the tire, a ratio P1 e/P1 c between a totalsum of ground-contact forces P1 c applied to a tire equator-side halfregion J1 c in the inner rib-like land portion J1 and a total sum ofground-contact forces P1 e applied to a ground-contact edge-side halfregion J1 e is set in a range of 0.8 to 1.0, a ratio P2 c/P1 e betweenthe total sum of ground-contact forces P1 e and a total sum ofground-contact forces P2 c applied to a tire equator side half region J2c in the outer rib-like land portion J2 is set in a range of 0.8 to 1.0,a ratio P2 e/P2 c between the total sum of ground-contact forces P2 cand a total sum of ground-contact forces P2 e applied to aground-contact edge side half region J2 e in the outer rib-like landportion J2 is set in a range of 0.6 to 1.0, wherein the total sums P1 c,P1 e, P2 c and P2 e have a relationship of P1 c>P1 e>P2 c>P2 e, and aratio P2 e/P1 c between the total sum of ground-contact forces P1 c andthe total sum of ground-contact forces P2 e is set in a range of 0,4 to0.9.